We present a hybrid quantum mechanical/molecular mechanical (QM/MM) model for microscopic solvation effects that includes polarizability in the MM region (QM/MMpol). QM/MMpol treatment of both ground and excited states is presented in the formalism. We present QM/MMpol analysis of the ground and electronic excited states of the bacteriochlorophyll b dimer (P) of the photosynthetic reaction center (RC) of Rhodopseudomonas viridis using the INDO/S method. We treat P and five adjacent amino acid side chains quantum mechanically, and the remainder of the protein, cofactors, and waters of crystallization with Polarizable MM (325 QM atoms embedded in the field of 20 158 polarizable MM atoms). While dimer formation alone is enough to account for the majority of the monomer BCh1b to P red-shift of the lowest electronic excited state of P (Q(y1)), we demonstrate that explicit treatment of the protein is required to properly interpret the experimental Stark effect data that describe the charge transfer asymmetry of Q(y1). The static-charge potential from the MM model of the RC alone causes Q(y1) to have significantly better agreement with the Stark effect results than isolated P. However, consideration of the protein polarization potential is further required to obtain more complete agreement with Stark effect experiments. Thus, we calculate a Q(y1) transition energy at 10 826 cm(-1) with a ground to excited state change in dipole moment of 4.8 D; an absorption Stark effect angle of 43 degrees; a net shift of 0.15 electrons from the L subunit to the M subunit of P; and a linear dichroism angle (between the transition moment of Q(y1) and the pseudo-C-2 axis of the RC) of 81 degrees. These results are in good agreement with experiment. Interestingly, we find that net CT increase is greater for Q(y1) than for the second excited state of P (Q(y2)), a result that we anticipated in an early model dimer study.